Surface proofing arrangement



June 20, 1967 G. STAPELFELD v SURFACE PROOFING ARRANGEMENT Filed Sept.30, 1964 United States Patent 3,326,001 SURFAtIE PROOFING ARRAVGEMENTGerhard Stapeifeld, Dortmund, Germany, assignor to GelsenkirchenerBcrgwerks-Aktien-Gesellschaft, Essen, Germany Filed Sept. 30, 1964, Ser.No. 400,377 Claims priority, application Germany, Oct. 3, 1963, G 38,84611 Claims. (Cl. 61-1) The present invention relates to a surfaceproofing arrangement and particularly to an arrangement for preventingthe seepage of a water-immiscible liquid below a given surface of aliquid-permeable material, such as the prevention of seepage of leak oilfrom stationary oil tanks into the ground so that such leak oil cannotreach the ground water level and cannot contaminate the ground water.

In order to maintain the purity of the ground water special measuresmust be taken to assure with certainty that no leak oil or other harmfulliquids will reach the ground water area. It has been proposed to forman impervious barrier for stationary oil tanks and the like of clay orsimilar material. However, this will give the desired result only if theunderlying ground is of suitable structure and it also requires theavailability of clay or other suitable loamy material for forming thebarrier. Only in relatively few locations can this type of barrier beproduced so as to give an effective protection and to be economicallyfeasible. Furthermore, it is not possible to control the effectivenessof the impervious layer or of individual portions thereof once the samehave been installed.

It has also been proposed to form the impervious layer of a foil orsynthetic material which is resistant against attack by crude oil. Suchfoil, on the one hand, must be hard and impact resistant and, on theother hand, resilient in order to withstand the stresses to which suchfoil will be exposed by uneven pressure of particles of the groundagainst the foil and by the movement of the ground. For these reasons,impervious layers consisting of a synthetic foil have met with onlylimited success and must be arranged with extraordinary care.

Rigid barriers such as forming a concrete layer in the ground orinserting concrete plates are not suitable because they can follow themovement of the ground layers only upon partial destruction of theconcrete or the like structure.

It is therefore an object of the present invention to provide a surfaceor ground proofing arrangement for preventing seepage of awater-immiscible liquid therethrough which will not be subject to theabove discussed difficulties and disadvantages.

It is a further object of the present invention to provide a surface orground proofing arrangement which can be produced in a simple andeconomical manner and which will effectively prevent undesirable seepagetherethrough.

It is yet a further object of the present invention to provide a surfaceor ground proofing arrangement which permits easy control of theimpermeability of individual portions thereof.

Other objects and advantages of the present invention will becomeapparent from a further reading of the description and of the appendedclaims.

With the above and other objects in view, the present inventioncontemplates a surface proofing arrangement for preventing seepage of awater-immiscible liquid below a given surface of a liquid-permeablematerial, comprising, in combination, a lower layer consisting of amaterial impermeable to liquids disposed to cover the surface, and

"ice

an upper layer superimposed on said lower layer and consisting of awater absorbent material, and of water absorbed in the upper layer andretained in the same by the lower layer so that the upper layer isadapted to repel the water-immiscible liquid whereby contact of the samewith the lower layer and seepage of the same into the liquid permeablematerial below the given surface is prevented.

Thus, according to the present invention the barrier is formed of apreferably highly capillary layer of synthetic foam plastic materialwhich layer is superposed upon an impervious foil or layer of syntheticplastic material. The porous capillary layer is capable of absorbingwater and has water absorbed in its capillary interstices and theunderlying impermeable synthetic foil prevents escape of the water whichis located in the porous foam plastic layer.

Preferably another foam plastic layer is arranged underneath theimpermeable foil and serves primarily the purpose of protecting thelower face of the impermeable foil against damage by possibly sharpedged particles of sand or other ground material.

In order to reduce the amount of synthetic material required for thefoam plastic layers and also to reduce the compressibility of the same,it is sometimes desirable to incorporate a filler material such asbentonite in the foam plastic layers and in such case the diameter ofthe individual particles of the filler material preferably should beless than 0.9 mm.

Within the two foam plastic layers located above and below theimpermeable foil, measuring electrodes may be arranged for the purposeof determining the location of any damaged and thus permeable portion ofthe impermeable foil. In order to control the water level above thebarrier layer, it is advantageous to provide a pumping arrangement atthe lowermost point of the trough formed by the barrier layer as will bedescribed in more detail further below.

The surface or ground proofing arrangement of the present invention isparticularly suitable for proofing the ground water carrying levelagainst penetration by leak oil from stationary oil tanks. However, theground proofing arrangement may be advantageously used wherever theproblem of preventing penetration of the ground water level by harmfulliquids may exist, particularly with respect to harmful liquids whichare immiscible with water and which have a lesser specific gravity thanwater, such as light fuel oil, medium heavy oil, gasoline, benzene,oxylene, p-xylene and similar products.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings, inwhich:

FIG. 1 is a diagrammatic cross sectional view through an oil tankinstallation provided with the ground proofing arrangement of thepresent invention; and

FIG. 2 is an enlarged cross sectional view through the portion of FIG. 1indicated by circle A and also includes a diagrammatic showing of atesting device for indicating any leakage through the ground proofingarrangement.

Referring now to the drawing it will be seen that a trough-shaped cavity14} is formed in the underground 11. The surface of the cavity is thencovered with a layer of sand 1 having a particle size of up to 3 mm.Sand layer 1 supports a highly capillary layer 2 of synthetic foamplastic material having a thickness of between about 3 and 5 cm. Layer 2may consist for instance of two layers, one

of foamed styrene and one of foamed urea formaldehyde resin. Layer 2supports a continuous, welded, liquid impervious foil 3 of syntheticplastic material and foil 3 is covered by a highly capillary syntheticfoam plastic layer 4 which, for instance, may be formed of foamed ureaformaldehyde resin. Layer 4 will be of such initial thickness that evenwhen covered by layer 5 and thus compressed the thickness of layer 4will still be at least about mm. The foamed layers may include fillermaterial having a particle size of less than 0.9 mm., for instance ofbentonite. The protective barrier which thus is formed of foamed layers2 and 4 with interposed impervious foil 3 is then covered with a sandlayer 5 having a thickness of about 50' cm. This sand layer 5 ispreferably compacted for instance by having a heavy caterpillar vehiclerun over the same. The sand of layer 5 preferably will have a particlesize of up to 3 mm. and is superposed by a surface layer 6 alsoconsisting of sand. A conventional pumping arrangement 7 is located soas to have suction pipes extending into the area of deepest depressionor the lowest point 12 of the cavity or trough 10.

After the sand layers which are superposed upon the plastic barrierlayers are compacted, the entire area of the thus filled cavity 10 iswatered preferably with about 10 to 15 liters of water per square meter.In this manner an artificial ground water level 9 is formed above thebarrier layers 2, 3, 4. The regular, natural ground water level isindicated by reference numeral 13. It will be seen that the regularground water will be maintained below the barrier layer 2, 3, 4. Oiltank 14 is located above the filled in cavity 10.

Foam plastic layer 2 will protect the superposed impervious foil 3against penetration and damage by sand particles. It is the purpose offoil3 to prevent downward escape of water from the highly capillarysuperposed water filled layer 4. Furthermore, porous foam layer 4 willalso protect foil 3 against damage by particles of sand or layer 5.Furthermore, due to its water content, layer 4 will serve to preventmigration of softeners from foil 3 and simultaneously also preventdirect contact between any leak oil or seepage from tank 14 and foil 3.Thus, the material which in fact prevents further downward passage ofthe oil is the water contained in foam plastic layer 4.

Throughout the entire barrier layer arrangement pairs of electrodes suchas the electrode pair 8 and theelectrode pair 8 are arranged with thetwo electrodes of each pair located adjacent each other in layer 2 andlayer 4 respectively. By connecting any given pair of electrodes to asource of electric current and to an instrument indicating the passageof current, it can be determined whether current flow takes placebetween'the two electrodes of the respective pair of electrodes. Suchcurrent fiow is an indication of water penetration from layer 4 throughfoil 3 into layer 2, in other words, an indication of damage orvariations in the height of the natural ground water level and that itcan also be easily located on sandy ground without risk of damage to theimpermeable foil 3 since the adjacent foam plastic layers 2 and4 willprotect foil 3 against damage by grains of sand. Since leak oil does notget into direct contact with foil 3, the washing out of softeners fromfoil 3is also effectively prevented. Due to the capillarity of layer 4,the same will contain absorbed water also in its more elevated portionssuch as those which are located within embankments 15.

The artificialground water level 9 above the barrier layer will becontrolled by operation of pumps 16 and 18 which are connectedrespectively to suction pipes 17 and 19. Suction pipe 17 reachessomewhat lower so as to be located within the uncontaminated waterlevel, while suction pipe 19 terminates at a somewhat higher point andmay deliver a mixture of water and leak oil. Due to its resiliency, thebarrier consisting of layers 2, 3 and 4 will easily and without damageadjust to movements of the ground.

Layers 2 and 4 may be formed of the same foam plastic material. However,it is essential that layer 4 consists of a hydrophilic material forinstance urea formaldehyde foam since the capillary interstices in layer4 are to be filled with water. With respect to layer 2 this is notabsolutely necessary and thus layer 2 may, for instance, be formed as acomposite layer comprising a core portion of styrene foam which iscovered on both sides by urea formaldehyde foam, or layer 2 may alsoconsist of a particularly stiff and rigid ureaformaldehyde foam sinceits main function is the cushioning of foil 3 while, in contrastthereto, layer 4 has the further function of being the carrier of thewater which in fact prevents contact between layer 3 and the leak oil orthe like.

When foaming urea formaldehyde, closed small bubbles of polyhedric shapeare formed which adhere to each other and which form between themselvesa system of capillary interstices which serve, as described above, forabsorbing and storing water in layer 4. It is the purpose of thismicroporous layer, due to its high capillarity, to maintain the water inlayer 4 even if the artificial ground water level 9 should drop. Inthiscase the capillarity of the layer will serve to hold the watertherein. The barrier water layer is essential in order to avoid wettingof foil 3 with leak oil which may originate fro-m tank 14. Contactbetween leak oil and water impermeable foil 3 particularly inconjunction with mechanical stress could lead to destruction of theimpermeability of layer 3.

Preferably the diameter of the capillaries forming a network ofinterstices in layer 4 should not exceed 0.01 mm. in order to assure thedesired capillary effect.

Microporous layer 4 and thus also microporous layer 2 may be formed ofpractically all available hydrophilic synthetic materials which can befoamed so as to form a network of capillary interstices and which willconform to the technical and chemical requirements. The microporous foamlayers must be capable to withstand chemical attack by the ground waterand should not be decomposed by organic solvents. Very good results areachieved with urea formaldehyde condensation products of whichcompressible foamed layers can be produced.

It is esseential that after hardening the foam layer can still becompressed and will not break when exposed to the load of the superposedsand layers.

To form porous layer 2 of polystyrene foam, for instance of typecommercially available under the tradename Styropor, and ureaformaldehyde foam has primarily economic advantages. By using Styroporfoil having a thickness of between 8 and 10 mm., it is possible toreduce the thickness of the urea formaldehyde foam layer so that thesame will be between 5 and 10 mm. in compressed condition. This willsuffice to protect foil 3 from mechanical attack by the underlying sandlayer.

Good results were obtained with an arrangement according to which onsandlayer 1 having a particle size of between 1 and 3 mm. is placed amicroporous, highly capillary hydrophilic synthetic foam plastic layerhaving a thickness in uncompressed condition of between about 20 and 25.mm. and consisting for instance of urea formalhyde foam. The nextportion of the layer consists of a Styropor foilor plate having athickness ofbetween about 8 and 10 mm. and this is followed by a secondmicroporous highly capillary hydrophilic synthetic foam layer of betweenabout 20 and 25 mm. thickness. The compound layer 2 consisting of. twomicroporous highly capillary hydrophilic synthetic foam layers with aninterposed Styropor layer should be so dimensioned that even under thegreatest load to which it will be exposed by the superposed sand layers5 and 6, layer 2 in thus compressed condition will still have athickness of at least mm. Thereby it is assured that foil 3 will not beexposed to mechanical attach from sand layer 1. Since Styropor possessesgreater rigidity than urea formaldehyde foam layers, it is possible touse in this manner a Styropor layer which is thinner than the ureaformaldehyde foam layer which would be required to replace it.

The incorporation of a filler material in the foamed layers 2 and 4serves for reducing the costs of the layers and also to improve themechanical resistance of the same. The amount of filler material such asbentonite should not exceed 50% of the weight of the layer and thefiller material must be evenly distributed throughout the layer with theindividual particles of the filler material having a size not exceeding0.9 mm.

Under operating conditions, i.e., when compressed by the superposed sandlayers 5 and 6, the thickness of each of layers 2 and 4 should not beless than 10 mm. respectively. The thickness of the uncompressed layeris best determined experimentally after the maximum height and thus theweight of the combined sand layers 5 and 6 has been established.

Urea formaldehyde foam with a Weight of between about 5 and kg./mpossesses the desired degree of plastic deformability. The volume ofcapillaries within the urea formaldehyde foam increases with increase inweight per volume of the same while simultaneously the size of theclosed bubbles decreases. Since a maximum capillary volume is desired,preferably a urea formaldehyde foam having a weight of between 10 and 15kg./m. will be used.

Plastic foil 3 should consist of a synthetic plastic material which isextensible even after prolonged storage and is capable of resistingchemical attack by the soil. Good results are achieved with foils ofpolyvinyl chloride, polyethylene and polyisobutylene. It is alsopossible to use such synthetic plastic foils which are coated with aspecial bitumen. In this case the synthetic plastic foil may be thinnerand thus more economical to use. In any event, it is essential that foilor layer 3 will remain impermeable to water even upon prolonged storageand upon being exposed to the movement of the underground. The thicknessof foil 3 should be at least 0.75 mm. and preferably will be about 1 mm.

Referring now again to FIG. 2 of the drawing, it is diagrammaticallyillustrated therein how the electrode pairs 8 and 8, which are onlyrepresentative of a large number of such electrode pairs, will serve tocontrol the impermeability of foil 3. As long as foil 3 is impermeableit constitutes an electric insulation between layers 2 and 4. In case ofpermeability, i.e., if the foil is punctured or torn at some place,water will penetrate from layer 4 through broken foil 3 into layer 2.Each electrode pair, such as electrode pair 8 or 8, consists of oneelectrode which is located in layer 4 which due to its water content isconductive and another electrode which is located in layer 2 which layeris insulated from layer 4 unless foil 3 is broken in the vicinity of theelectrode pair. As illustrated in FIG. 2, the electrical circuit will beclosed only when Water has penetrated into layer 2 and thus has madelayer 2 conductive. The thus established current flow can then beindicated in conventional manner at 20. Preferably, the electrodes arelocated close to the adjacent sand layers 1 and '5 respectively. It hasbeen found advantageous to use electrodes of synthetic resinbound carbonwhich are then connected to the required wiring because in this mannercorrosion and the formation of an electric cell is effectivelyprevented.

Each pair of electrodes is separately connected with the control devicesand preferably voltage and current flow will be shown by the indicator.

The distance of the electrode pairs from each other may be between 50and meters depending on how accurately one wishes to determine the pointat which foil 3 has become permeable. If sand layer 6 is relativelythin, the electrode pairs may be spaced further apart since the costs ofdigging to the foil will be relatively small. On the other hand, if sandlayer 6 is very thick, it will be economical to arrange a greater numberof electrode pairs so as to determine with greater accuracy the area inwhich foil 3 will have to be repaired.

As illustrated in FIG. 1, two pumps are arranged with suction pipesextending in a conventionally structured sump to different levels in thevicinity of the lowermost portion of the trough formed by the barrierlayers 2, 3 and 4. Preferably water is removed through suction pipes 17and pump 16 only, until the water level in the sump has been lowered tothe vicinity of the outlet end of suction pipe 19 and only then anoil-water mixture is withdrawn through suction pipe 19 and pump 18.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofsurface proofing arrangements differing from the types described above.

While the invention has been illustrated and described as embodied in aground surface proofing arrangement for preventing seepage of awater-immiscible liquid below a given ground surface, it is not intendedto be limited to the details shown, since various modifications andstructural changes may be made without departing in any Way from thespirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A ground surface proofing arrangement for preventing seepage of awater-immiscible liquid below a given ground surface, comprising, incombination, a lower layer consisting of a water-impermeable sheet ofplastic material disposed to cover said ground surface; and an upperlayer superimposed upon and contacting said lower layer and consistingof a water absorbent foam plastic material, and of water absorbed insaid upper layer and prevented from escaping downwardly by said lowerlayer.

2. A ground surface proofing arrangement as defined in claim 1, whereinsaid water absorbent foam plastic material of said upper layer consistsessentially of a urea formaldehyde resin.

3. A ground surface proofing arrangement for preventing seepage of awater-immiscible liquid below a given ground surface, comprising, incombination, a trough-shaped lower layer consisting of a liquidimpermeable sheet of plastic material disposed to cover a depressedportion of said ground surface; and an upper trough-shaped layer havinga configuration matching said lower layer and superimposed on the sameto cover the entire top surface of the same, said upper layer consistingof a water absorbent synthetic foam plastic material, and of waterabsorbed in said upper layer and retained in the same by said lowerlayer so'that said upper layer is adapted to repel said water-immiscibleliquid whereby contact of the same with said lower layer and seepage ofthe same into the ground below said given ground surface is prevented.

4. A ground surface proofing arrangement for preventing seepage of awater-immiscible liquid below a given ground surface, comprising, incombination, a trough-shaped lower layer consisting of a liquidimpermeable sheet of plastic material disposed to cover a depressedportion of said ground surface; and an upper trough-shaped layer havinga configuration matching said lower layer and superimposed on the sameto cover the entire top surface of the same forming a cavity open ontop, said upper layer consisting of a water absorbent synthetic foamplastic material; a water absorbent material at lease partially fillingsaid cavity; and water absorbed in said water absorbent material in saidcavity and in said upper layer and retained in the latter by said lowerlayer so that said upper layer. is adapted to repel saidwater-immiscible liquid whereby contact of the same with said lowerlayer and seepage of the same into the ground below said given groundsurface is prevented.

5. A ground surfact proofing arrangement for preventing seepage of awater-immiscible liquid below a given ground surface, comprising, incombination, a bottom layer consisting of a foam plastic materialdisposed to cover said ground surface; an upper layer superimposed onsaid bottom layer and consisting of a water absorbent foam plasticmaterial and of water absorbed in said upper layer; and alayerconsisting of a waterimpermeable sheet of synthetic plastic materialinterposed between and contacting said bottom and said upper layer andpreventing contact between said water in said upper layer and said lowerlayer so that said upper watercontaining layer is adapted to repelwater-immiscible liquid whereby contact of water-immiscible liquid withsaid water-impervious sheet and seepage of water-immiscible liquid intosaid ground surface is prevented.

6. A ground surface proofing arrangement for preventing seepage of awater-immiscible liquid below a given. ground surface, comprising, incombination, a bottom layer consisting of a foam plastic materialdisposed to cover said ground surface; an upper layer superimposed onsaid bottom layer and consisting of a water absorbent capillary,filler-containing foam plastic material and of water absorbed in saidupper layer; and a layer consisting of a water-impermeable sheet ofsynthetic plastic material interposed between and contacting said bottomand said upper layer so as to prevent contact between said water in saidupper layer and said lower layer so that said upper water-containinglayer is adapted to repel water-immiscible liquid.

7. A ground surface proofing arrangement for preventing seepage of awater-immiscible liquid below agiven ground surface, comprising, incombination, a bottom layer consisting of a filler-containing foamplastic ma-- terial disposed to cover said ground surface; an upperlayer superimposed on said bottom layer and consisting of a waterabsorbent foam plastic material and of water absorbed in said upperlayer; and a layer consisting of a water-impermeable sheet of syntheticplastic material interposed between and contacting saidbottorn and saidupper layer so to prevent contact between said .water in said upperlayer and said lower layer so that said upper water-containing layer isadapted to repel waterirnmiscible liquid.

8. A ground surface proofing arrangement for preventing seepage of awater-immiscible liquid below a given ground surface, comprising, incombination, a bottom layer consisting of a foam plastic materialcontaining a particulate filler having a particle size below 0.9 mm.disposed to cover said ground surface; an upper layer superimposed onsaid bottom layer and consisting of a water absorbent foam plasticmaterial of capillary structure and containing a filler material havinga particle size of below 0.9 mm. and of water absorbed in said upperlayer; and a layer consisting of a water-impermeable sheet of syntheticplastic material interposed between and contacting said bottom and saidupper layer so as to prevent contact between said water in said upperlayer and said lower layer so that said upper water-containing layer isadapted to repel water-immiscible liquid.

9. A ground surface proofing arrangement for preventing seepage of awater-immiscible liquid below a given ground surface, comprising, incombination, a

o trough-shaped lower layer consisting of a liquid impermeable sheet ofplastic material disposedto cover a depressed portion of said groundsurface; and an upper trough-shaped layer having a configurationmatching said lower layer and superimposed on the same to cover theentire top surface of the same forming a cavity open on top, said upperlayer consisting of a water absorbent synthetic foam plastic material;sand at least partially fill ing said cavity; and water absorbed in saidsand in said cavity and in said upper layer and retained in the latterby said lower layer so that said upper layer is adapted to repel saidwater-immiscible liquid whereby contact of the same with said lowerlayer and seepage of the same into the ground below said given groundsurface is prevented.

10. A surface proofing arrangement for preventing seepage of awater-immiscible liquid below a given surface of a liquid permeablematerial, comprising, in combination, a bottom layer consisting of afoam plastic material disposed to cover said surface; an upper layersuperimposed on said bottom layer and consisting of a water absorbentfoam plastic material and of water absorbed in said upper layer; a layerconsisting of a liquidimpervious sheet of synthetic plastic materialinterposed between and contacting said bottom and said upper layer andretaining said water in said upper layer so that said upper layer isadapted to repel said water-immiscible liquid whereby contact of thesame with said liquid impervious sheet and seepage of the same into saidbottom layer is prevented; and electrode means operatively connected tosaid bottom layer and said upper layer and including current supplymeans and indicating means for determining current flow between saidupper and said bottom layer indicating rupture of said passage of waterthrough said interposed impervious sheet.

11. A ground surface proofing arrangement for preventing seepage of awater-immiscible liquid below a given ground surface, comprising, incombination a trough-shaped lower layer consisting of a liquidimpermeable sheet of plastic material disposed to cover a depressedportion of said ground surface; and an upper trough-shaped layer havinga configuration matching said lower layer and superimposed on the sameto cover the entire top surface of the same forming a cavity open ontop, said upper layer consisting of a water absorbent synthetic foamplastic urea formaldehyde resin; sand at least partially filling saidcavity; and water absorbed in said sand in said cavity and in said upperlayer and retained in the latter by'said lower layer so that said upperlayer is adapted to repel said water-immiscible liquid whereby contactof the same with said lower layer and seepage of the same into theground below said given ground surface is prevented.

References Cited UNITED STATES PATENTS 2,437,909 3/1948 Cooper 615 X2,794,756 6/ 1957 Leverenz. 2,806,812 9/1957 Merz. 3,004,293 10/ 1961Kreidl.

3,142,855 8/1964 Gilchrist 161159 X 3,188,814 6/1965 Rettiug 6131FOREIGN PATENTS 1,267,566 6/ 1961 France.

789,644 1/1958 Great Britain.

OTHER REFERENCES Abstract 148,996, O.G., vol. 656; page 893; Mar. 18,1952.

Popular Mechanics; April 1963, page 126.

EARL J. WITMER, Primary Examiner.

1. A GROUND SURFACE PROOFING ARRANGEMENT FOR PREVENTING SEEPAGE OF AWATER-IMMISCIBLE LIQUID BELOW A GIVEN GROUND SURFACE, COMPRISING, INCOMBINATION, A LOWER LAYER CONSISTING OF A WATER-IMPERMEABLE SHEET OFPLASTIC MATERIAL DISPOSED TO COVER SAID GROUND SURFACE; AND AN UPPERLAYER SUPERIMPOSED UPON AND CONTACTING SAID LOWER LAYER AND CONSISTINGOF A WATER ABSORBENT FOAM PLASTIC MATERIAL, AND OF WATER ABSORBED INSAID UPPER LAYER AND PREVENTED FROM ESCAPING DOWNWARDLY BY SAID LOWERLAYER.